Modern computer systems often are assembled by interconnecting various electronic modules on printed circuit boards. The printed circuit boards are commonly loaded into racks or "card cages," which facilitate the mounting and connection of the electronic modules. Card cages are a form of housing for supporting printed circuit boards in compact or dense configurations. Card cages typically comprise an open framework with a top, bottom, back, and sides. Front support rails define an open front through which the printed circuit boards are inserted into the card cage. The back typically supports a backplane with backplane connectors for receiving mating connectors on a rear edge of each printed circuit board. Rails attached to the top and bottom or to the sides or intermediate locations engage parallel edges of the printed circuit board to guide the printed circuit board into the card cage in either a vertical or horizontal orientation, to align the printed board connector with a corresponding backplane connector, and to support the printed circuit board after it is seated in the card cage.
It is often desirable to lock the printed circuit board in place. For example, commercial underwriters impose requirements on manufacturers of systems that have printed circuit boards operating at voltages above a certain level. The manufactures must provide safety mechanisms that prevent individuals from accidentally accessing the printed circuit boards.
In the past, manufacturers used machine screws that lead through lead holes in the front panel. These screws thread into a front support rail to provide the locking function required by the commercial underwriters. This approach, however, required that an individual take additional time to thread and unthread the screws through the front support rail whenever a printed circuit board was put in or taken out of the card cage. It was also possible for individuals to bypass the locking function by simply not threading the screws at all.